1. Field of the Disclosure
This disclosure generally relates to catalysts, and to electrodes comprising those catalysts, for use in fuel cells. More specifically, this disclosure relates to catalysts active towards hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) and to porous electrodes which are made in a process designed to control their porosity, employed in regenerative fuel cells, comprising hydrogen and halogen acid or mixture of halogen acids. The catalysts are particularly useful in hydrogen/bromine reduction/oxidation reactions. The catalysts exhibit highly acceptable life and performance.
2. Discussion of the Background Art
A typical fuel cell consists of two electrodes, an anode and a cathode, and a membrane interposed between the anode and cathode. Fuel cells operate by converting fuel combustion energy, such as hydrogen, to electrical power through an electrochemical process. It does so by harnessing the electrons released from controlled oxidation-reduction (redox) reactions occurring at the surface of a catalyst dispersed on the electrodes.
A commonly used catalyst, as a result of its stability in harsh environments of regenerative fuel cells, is nanometric platinum supported on carbon black. An important issue connected to catalyst activity in regenerative fuel cells that utilize, for example, a halogen acid electrolyte, is poisoning of the hydrogen catalyst by the halides. The membrane cannot completely prevent electrolyte crossover from one side of the cell to the other. For example, in a hydrogen tri-bromide fuel cell (HTBFC), bromides, e.g., tri-bromide, diffuse to the hydrogen electrode and poison the catalyst. Despite the fact that hydrogen oxidation/evolution reaction is fast and its overpotential is rather low compared to other voltage losses in the regenerative cell, in halogen ion-containing solutions, the catalyst is severely poisoned, and this raises the overpotential of the hydrogen electrode in the regenerative fuel cell.
In fuel cells, porous electrodes are typically encountered because of the high power density and in electrolyzers because of the high rate of chemicals production per unit area. A problem common to porous electrodes is to provide the most effective pathway throughout the electrode for each reactant and product involved in the electrochemical reaction, and enhance a surface area between the active material and the electrolyte, making it as large as possible. A major drawback of porous electrodes is flooding of the electrodes. Water formed can be transported through the membrane together with protons, filling the electrode pores and preventing gas from reacting.
Acceptance of a regenerative fuel cell as a viable energy source depends on its cycle life. Regenerative fuel cells can be run, in addition to the direct mode, in the reversible mode, consuming electricity and the products of the direct reaction in order to produce the reactants of the direct reaction. For regenerative fuel cell such as hydrogen/bromine fuel cells, an important factor limiting its cycle life is the degradation of the operating fuel cell materials. These materials are exposed to a highly corrosive bromine electrolyte for long periods of time at elevated temperature.
As indicated above, bromide, e.g., tri-bromide, diffusion through the membrane from the solution to the gas electrode limits the cycle life. Ideally, the membrane would only transport protons or cations and exclude anions. Under ideal conditions in a hydrogen/bromine fuel cell, the hydrogen catalyst will only be exposed to gaseous and ionic hydrogen. In reality, the membranes do, not completely prevent bromide ions from passing through the membrane and adsorbing on the hydrogen electrode. As a result of this, the hydrogen/bromine fuel cell cycle life becomes limited because of several reasons including catalyst corrosion and poisoning of the hydrogen catalyst.
A need exists for catalysts that exhibit stability in harsh environments of regenerative fuel cells, in particular, hydrogen/halogen fuel cells. Also, a need exists for catalysts that are capable of catalyzing both charging and discharging reactions in a regenerative fuel cell, in particular, a hydrogen/halogen fuel cell. Further, a need exists for catalysts that are capable of catalyzing both HERs and HORs in a regenerative fuel cell, in particular, a hydrogen/halogen fuel cell. It would be desirable in the art to provide catalysts for regenerative fuel cells having low cost and acceptable life and performance.